So I thought to quiet my PS a bit I would replace the 80mm fan thermistor with a potentiometer. I got a 10K "volume control" from RadioCrap since they didn't have any 100K ones that I could see.

The first attempt seemed to work pretty well: when I cut off the termistor, the fan stopped. When I twisted the wires together the fan when into Concorde-engine-mode. When I attached the wires to the potentiometer, it started again. If I turn the "volume" all the way up, the fan makes a huge racket, and I can turn it all the way to off and the fan stops.

I used the center post and the rightmost post (looking at it from the back like this picture)

Now I thought I'd get really smart and put the thermistor in series with the potentiometer. This way I can set it to be quiet when the load is low, and then if things heat up cause 16 people log into my Quake III server and I'm not around it can still speed up.

This where it gets wierd: Now the fan doesn't turn off when I turn the volume all the way down. Now I'm no electronics whizz, but when you put resistors in series doesn't their resistance get added? And if the volume is down, the resistance is infinite right? Inifinity plus whatever the thermistor adds should still be infinity.

Anyway let me boil this down to the questions:

1. What are those three tabs on the potentiometer for?

2. My potentiometer has three tabs on the top, and three more on the back. Why?

3. Whats with putting the potentiometer and thermistor in series?

4. Is there another arrangement of these two that will do what I want?

5. If the potentiometer is basicly a variable resistor, and goes from "off"/infinite resistance to "full volume"/no resistance then what does that 100KOhm, 1000KOhm number represent?

I keep meaning to learn some electronics, but I never have time to read a book...

From the perspective of an electronics technician, following are several notes to reply to your message. There is no straight-forward answer here. So I will try and keep the techno-jargon to a minimum, unfortunately it will not be a short message.

You are correct that a potentiometer is a varible resistor and is (usually) a 3-terminal device. The total resistance of the potentiometer is between the two outside terminals (I.E. # 1 and 3) and the middle terminal is the variable wiper (I.E. # 2).

If the potentiometers resistance is 100Kohm (K = 000, therefore 100K = 100,000-ohms) and you measure with a volt-ohm meter (for resistance) across pins 1 & 3, you should measure around 100,000-ohms (+/- tolerance of potentiometer). However, if you measure between pins 1 & 2, and adjust the potentiometer, you will see that from one end of the scale to the opposite end, the resistance will change accordingly (0 to 100,000 or vice-versa).

If you have a potentiometer with more than 3-terminals, it could be a stacked or twin layered device for a stereo volume or balance control. If you have a terminal on the opposite side of the potentiometer from the 3-terminals, this is usually a mid-span "tap" used in audio applications (no need to cover here). If you have terminals on the back surface, these could be the on/off switch inside the potentiometer.

BTW - a potentiometer in a seteo amplifier works quite differently than in a PSU fan control circuitry. In a stereo, at the low volume settting, the resistance is zero (zero volts of audio) and at the high volume setting, the resistance in maximum (higher audio voltages). his can easily be seen with an aid of an oscillscope.

When you cut the thermistor from the circuit, the fan control circuit was basically turned off and when you connected the wires together, the fan would receive full voltage from the control circuit. Good this tells me the control circuit & fan still works.

BTW the fan control circuit is "normally" a dual-stage circuit. In other words, a power source, 2 transistors, a few resistors, the thermistor and the fan.

Thermistors usually have a negative temperature coefficient. In other words, the cold resistance is more than the hot resistance. The temperature around the thermistor varies the resistance in the thermistor, thereby affecting the current through the device. The thermistor controls the fan circuitry and when the thermistor is hot, the fan turns faster - thermistor is cold, the fan slows down. Simple really.

When you installed the potentiometer, the fan started to work. At one end of the scale, the potentiometer provided "zero" resistance and the fan received zero voltage and the fan would be off. At the opposite end of the potentiometer scale, the resistance will be low and the fan will make the "Concorde-mode" racket. This potentiometer needs to mimick the thermistor (negative temperature coefficient).

I would have predicted the results as you had received with the potentiometer. BTW the picture provided is correct for installation of the potentiometer. Now comes the tricky stuff. You said you had placed the thermistor in series with the potentiometer and things got weird. Remember the negative temperature coefficient? That is going to throw a wrench into your circuit.

With the potentiometer at the lowest setting (zero resistance) the thermistor will be the only resistance in the circuit. Then when the thermistor is hot, the thermistor resistance drops and the fan should run faster (same as it had originally before this mod). However, if the potentimeter is set to the opposite end (high resistance) and the thermistor heats up, I doubt whether the fan will even start up.

What I would have done is different. Lets assume the thermistor is 100,000-ohms. I would have chosen a potentiometer around 500,000 to 1M-ohms (1,000,000-ohms) and intall in in parallel to the thermistor.

Your statement that resistors in series are additive is correct. But in a parallel configuration, the resistance can be cut in half (assuming equal resistances). In the case of a 100K-ohm thermistor in parallel with a 500 to 1M-ohm potentiometer, the lowest resistance will be slightly lower than 100K-ohm. This should not pose a problem here.

At one end of the potentiometer scale, the fan would be driven hard (on) and the thermistor would have no effect. At the opposite end of the potentiomenter, the potentiometer is mostly out of the circuit and the thermistor would be the automatic control function. To adjust for better fan control at rated temperatures, you would need to play with the potentiometer to find that happy medium.

Whew!

At this point, further examination and evaluation of the fan / thermistor / potentiometer control circuitry needs to be done. It is difficult for me to provide an end-all, beat-all idea without first trying it.

When you cut the thermistor from the circuit, the fan control circuit was basically turned off and when you connected the wires together, the fan would receive full voltage from the control circuit. Good this tells me the control circuit & fan still works.

TerryW wrote:

Thermistors usually have a negative temperature coefficient. In other words, the cold resistance is more than the hot resistance. The temperature around the thermistor varies the resistance in the thermistor, thereby affecting the current through the device. The thermistor controls the fan circuitry and when the thermistor is hot, the fan turns faster - thermistor is cold, the fan slows down. Simple really.

But doesn't the fact that separating the wires (thereby creating "infinite" resistance) stops the fan, and twisting the wires together (thereby creating "zero" resistance) runs the fan at full speed imply that the thermistor has a positive termperature coefficient? Less resistance seems to equal faster fan rotation, therefore higher heat would result in higher resistance.

Unless, that is, separating and shorting the thermistor wires are cases that "disable" and "confuse" the fan control circuit respectively. That would be strange, since if the PS got cold enough to make the thermistor's resistance drop to zero, the fans would suddenly kick into high gear.

To the best of my knowledge, thermistors have negative temperature coefficients (this is supported by data books I reviewed). I suppose a manuacturer could have come up with a thermistor with a positive coefficient, I just have not heard of one. Also, thermistors have a resistance range to match a specific temperature range and quite possibly will never acheive zero resistance.

FYI - A solid-state sensor (AD590) generally has a positive temperature coefficient and is usually linear through its temperature range. Some solid-state thermosmeters use this device for their sensor.

As I had mentioned in my earlier reply, the fan control circuit is a 2-stage and is usually an emitter-follower design. Ths means that whatever the input does (thermistor up or down resistance) the output follows (fan high to low speed).

In other words, you remove the thermistor (open wire leads), the fan is off. Short the wire leads, fan is in Concord-mode. A potentiometer will still vary the fan control circuit, thereby the fan speed.

Generally, thermistors are high resitance (10K to 100K ohms) and the fan is low impedance (50 to 200 ohms). If the thermistor was wired in series with the fan, the fan would not receive enough current through the thermistor to turn the fan on.

More tehno-jargon to follow.

Ohms Law:

Ohms Law is very simple: a simple formula can define what the Voltage (E), Resistance (R) and/or Current (I) will be. If you have two of the 3 elements (E & R), you can determine the third element (I). As an example, E divided by R = I; E divided by I = R; I times R = E. The rest of the Law can be overwhelming and I won't bore you with those details.

Therefore, we know the voltage (12VDC) and the thermistor resistance (say 40K-ohms @ hot). The fan operating current @ 0.12-Amperes (120 milliamperes), but is not required as part of the formula.

If we were to simply place the thermistor across a 12VDC source and have the thermistor hot (@ say 40,000-ohms), Ohms Law defines the current through the resistor as being this equation: 12(VDC) divided by 40,000 (ohms) = 0.0003 amperes (0.3 milliamperes). As you can see if the thermistor was wired in series with a device that requires 120-milliampers, the fan would never turn on.

This is why we need a transistorized fan control circuit.

I do have a ATX PSU schematic that I could provide, and if you were interested, I could provide a detailed explanation of the fan control circuitry.

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